The earth is now facing serious problems like global warming and desertification. In recent years, rapid increases in the global population, decreases in crop yield due to unusual weather, and the like have raised concerns about a major, worldwide food crisis. Increasing productivity by improving growth of crops and plant growth in tree plantation has been considered one of the possible effective strategies to solve such problems.
Chemical fertilizers, agrichemicals and the like have been developed to improve plant growth, and are conventionally in practical use. However, in consideration of their effects on the plants themselves, and their effects on the human body, the usage amount of chemical fertilizers and agrichemicals must be restricted. Particularly in the harvesting of crops, chemical-free farming has now increased in order to reduce the adverse effects on human body, even at the expense of productivity.
As a technology to improve plant growth without using chemical fertilizers or agrichemicals, some methods for enhancing plant root elongation using genetic engineering have been reported. For example, Non-Patent Document 1 reports that overexpressing a cyclin gene in plants result in enhancing root elongation and growth in the parts above ground.
Ran protein is known as a factor involved in important cellular phenomena in plants, such as nuclear membrane-mediated protein transport, formation of microtubules during mitosis, cell cycle control, and the like. In recent years, the suppression of root elongation (see Non-Patent Document 2) through the introduction of a wheat-derived Ran gene into rice or Arabidopsis was reported. Based on this, the introduction of the Ran gene into plants has been thought to adversely affect plant growth.
Furthermore, even though it is well known that plant root in dry soil, hypersaline soil or the like is exposed to hyperosmotic stress and therefore have their elongation suppressed, no means of stably elongating roots under such hyperosmotic stress have been known.    Non-Patent Document 1: Doerner et al., (1996) Nature, 380: 520-523    Non-Patent Document 2: Xin Wang et al., (2006) Plant Physiology, 140: 91-101